The polymerization of cyclic diorganosiloxanes to high molecular weight polymers requires that the monomer first be dried. It is preferred that the moisture level be reduced to a level which minimizes the amount of silanol end groups formed on the polymer chain ends. Evans, U.S. Pat. No. 4,122,247 discloses a process for the polymerization of cyclic diorganosiloxanes with cation-complex catalysts in which it is preferred that there be present less than 10 ppm of water in the composition of cyclic siloxanes which is to be used with the catalyst to make the polymers. Removal of all but traces of water is accomplished by heating to 100.degree. C. or above with a nitrogen purge. According to the patent, this effectively reduces the water content of the cyclic siloxane composition to less than 10 ppm. If there is substantially more than this amount of water present in the cyclic siloxanes, then the desired low molecular weight oil or high molecular weight diorganosiloxane homo or copolymer gum will not be formed in commercially attractive yields. In essence, Evans raises the temperature of the material above the boiling point of water to drive off the excess water by distillation.
Saam et al., U.S. Pat. No. 4,780,519 discloses a method of producing polydiorganosiloxane gum in which azeotropic drying is employed to reduce the water level. This is accomplished by controlling the water vapor pressure in the reaction container. In particular, the reaction container is sealed so that the atmosphere can be controlled at a desired water vapor level. According to Saam, water vapor pressure is regulated by means of water vapor control of a nitrogen sweep during polymerization. A dew point meter placed at the exit stream of the reaction container is used to determine the amount of water vapor pressure present in the reaction container. By control of the dew point, that is, by the control of the amount of water carried by the nitrogen into the vessel, the amount of water in the reaction can be controlled. Saam also mentions that distillation techniques are available but not satisfactory.
The various methods discussed in Saam are deficient in a number of ways. Drying and polymerization occur simultaneously. Thus, the heating step may cause premature polymerization which can result in poor product yield. The various methods require additional apparatus to achieve the desired results. Further, unless the variables such as, for example, time, temperature and dew point are strictly controlled, such known processes may not achieve satisfactory results.
The moisture content in the reaction is controlled to minimize the amount of silanol end groups and to control the desired or target molecular weight range. The nature of the end groups and the molecular weight are controlled by the addition of chain stoppers such as: EQU HO-Si(R.sub.1)(R.sub.2)-O-[siloxane chain]-Si(R.sub.1)(R.sub.2)-OH, EQU (R.sub.1).sub.3 Si-O-[siloxane chain]-Si(R.sub.1).sub.3,
or EQU (R.sub.4)Si(R.sub.1).sub.2 -O-[siloxane chain]-Si(R.sub.1).sub.2 (R.sub.4),
where
R.sub.1 is CH.sub.3 ; PA1 R.sub.2 is CH.sub.3 or CF.sub.3 --(CH.sub.2).sub.2 --; and PA1 R.sub.4 is CH.sub.2 =CH--.
It is known that high molecular weight (HMW) polymers can be derived without drying from cyclic siloxanes, in particular from 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl) cyclotrisiloxane to provide gums with a Williams Plasticity (WP) (3') of &lt;200. The value can be increased by azeotropically drying the polymer prior to polymerization. However, azeotropic drying is inefficient and does not result in the desired degree of control of the resulting high molecular weight range. A narrow range of molecular weight has a positive effect upon the producibility and quality of resulting compounds, for example heat cured rubbers. Distillation and azeotropic drying, while effective to produce a water level thought to be necessary in this art, nevertheless result in shortcomings, both in terms of cost and quality of the product.